Distinguishing human-generated input from programmatically-generated input

Provided is a process, including: sending, via a first channel, to a client computing device, a challenge of a test to determine whether a human or a computer is generating input; receiving, via a second channel, a response to the challenge, wherein the second channel and the first channel are not both accessible to a client-application in which the response is input, and wherein the test includes an image recognition task by which the response is determined from the challenge; and determining whether a human or computer generated the response based on performance in the image recognition task indicated by the response.